Optical coherence tomography (OCT) is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). The principle of ocular imaging with OCT is based upon measuring the time-delay of the light reflected from each optical interface (A-scan) when a pencil of light enters the eye. A series of A-scans across the structure permits a cross-sectional reconstruction of a plane through the anterior or posterior segment of the eye. This is known as a B-scan.
Speckle noise is problematic for OCT imaging as well as other imaging modalities including synthetic aperture radar (SAR), remote sensing, ultrasound, sonar, etc. In these fields, a substantial amount of signal processing research has been conducted to combat speckle noise, resulting in the development of powerful digital filters for reducing speckle while preserving edge sharpness. Speckle reduction in OCT has been addressed by modifying imaging configurations and through the use of signal processing and image filtering. However, these methods often remove some details of the images and are unsuitable for OCT images in medical applications.
In typical OCT imaging, a single frame or slice of the image usually has very poor quality due to large speckle noise. Therefore, multiple scans are obtained and image averaging is applied, i.e., an average image is computed from the multiple image slices.
An example of an OCT system is the Topcon DRI OCT-1, a swept source OCT for posterior imaging, utilizing a wavelength of 1,050 nm. It has a scanning speed of 100,000 A-scans/sec. Utilizing this 1,050 nm wavelength, DRI OCT-1 can penetrate deeper compared to the current conventional OCTs with wavelength in the 850 nm range. Visualizing ocular tissues such as choroid or even sclera, can be done within a very short time. Deep range imaging DRI OCT-1 displays detail structures of not only retina but choroid and even sclera. With the capability of imaging deeper anatomic structures with less influence in scattering, DRI OCT-1 can visualize the entire tomogram with high sensitivity.
Depending on different applications and resolutions, different modes of scanning are provided in DRI OCT-1 including the line mode, 5 lines cross mode, 12 radial mode, circle line mode and 3D horizontal/vertical mode. In line mode, a line scan is conducted and repeated for a maximum of 96 times. In 5 line cross mode, 5 horizontal line scan and 5 vertical line scan are conducted with each line scan repeated for a maximum of 32 times. In 3D mode, a current swept source machine allows 4 repeated scans in 3D vertical mode with resolution 512×64 while each location is scanned only once in 3D horizontal model with 512×256 resolution. Since these scans are obtained from one location, they are averaged to get the final result with some help of image registration to overcome possible eye movement between different scans.